PEL: set pdbg environment variable
PEL: set pdbg environment variable

pdbg needs PDBG_DTB environment variable initialisation
prior to pdbg init to set correct device tree back-end.

Tested: After software update with this change verified
        that pdbg is using BMC initialised DEVTREE.

Signed-off-by: Jayanth Othayoth <d8c0d7e9f313f85e22ced997a3958fcdeb1cb606@in.ibm.com>
Change-Id: I6a59a79ccd9ae1f7d5947df0905e4ba999f835ec

openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging,openbmc/phosphor-logging